In the prior art, in an image reading apparatus which is used in a copier, a scanner, a multi-function apparatus having a copying function and a scanning function, or similar apparatus, the image reading apparatus includes an automatic document feeder (ADF) which feeds a document from a sheet feed tray to a sheet discharge tray through a feed path. In order to read both the first and second sides of a document of which images are printed, a document feeder bidirectionally feeds the document while it is being fed, to thereby reverse the leading and trailing ends of the document and thus scan both sides of the document (for example, see JP-A-8-85649).
FIG. 45 is a typical view of a feed path used in a existing document feeder capable of scanning (reading) both sides of a stack of documents. As shown in FIG. 45, a stack of documents P placed on a sheet feed tray 100 with the first side (first page) thereof facing upward is fed to a feed path 102 by a sheet feed roller 101. In the feed path 102, the document P is fed to feed rollers 103 which are disposed properly according to this situation. When the document P passes through a scanning position X, the first side of the document P is scanned by an image read unit such as a CCD or a CIS. When a sensor detects the trailing end of the document P after the first side thereof is scanned, sheet discharge rollers 104 are caused to stop in a state where the sheet discharge rollers 104 nip the vicinity around the trailing end of the document P.
As shown in FIG. 46, the reversible rollers 104 reverse the transfer direction of the nipped document P to a return path 105. The document P is fed from the return path 105 again to a side upstream of the scanning position X. As a result, the leading and trailing ends of the document P are reversed. Then the document P is fed by the feed rollers 103, and while being passing through the scanning position X, the second side of the document P is scanned by the image read unit. When a sensor detects the trailing end of the document P after the second side of the document P is scanned, the sheet discharge rollers 104 are again stopped in a state where the vicinity around the trailing end of the document P is nipped. Afterwards, the document P is sent back along the return path 105. When the document P is moved from the return path 105 again into the feed path 102, the document P is held in a state where the leading and trailing ends of the document P are reversed once more, that is, the first side of the document P is opposed to the scanning position X. The document P is transferred along the feed path 102 and is discharged into a sheet discharge tray 106 with the first side thereof facing downward. As a result, both the first and second sides of the document P are scanned and the document P is discharged to the sheet discharge tray 106 in the order that the sheets of the document P were stacked on the sheet feed tray 100.
The sheet feed roller 101, feed rollers 103, and sheet discharge rollers 104 are rotated in given directions by being transmitted with a drive force from a motor. The sheet feed roller 101 and feed rollers 103 are always rotated in one direction, that is, in a direction to feed the document P from upstream to downstream of the feed path 102. On the other hand, the sheet discharge rollers 104 are rotated in both forward and backward directions to carry out a switchback feed. For example, as shown in FIG. 46, when the document P is nipped between the feed rollers 103 and sheet discharge rollers 104, it is necessary that a document feed direction of feed rollers 103 and a document feed direction of the sheet discharge rollers 104 are coincident with each other. In addition, when the document is nipped by the feed rollers 103 on the immediate upstream side of the scanning position X and the sheet discharge rollers 104, it is necessary that a document feed direction of the feed rollers 103 and a document feed direction of the sheet discharge rollers 104 are coincident with each other. Therefore, for example, a motor to drive the feed rollers 103 and a motor to drive the sheet discharge rollers 104 are separately provided, the feed rollers 103 are always rotated in one direction, and sheet discharge rollers 104 are switched in the rotating direction at a given timing. In addition, an electromagnetic clutch is provided so as to transmit drive to the sheet feed roller 101, the feed rollers 103, the sheet discharge rollers 104, etc., from a single motor and cut drive force to rollers uninvolved in feeding of the document P at a predetermined timing.